In the process industry it often happens that pipes used to transport products between the various stages of the process are subject to varying temperatures. An increase in temperature causes pipes to expand and attempt to increase in length, and a decrease in temperature has the reverse effect. If the pipes are rigidly mounted then the forces resulting from temperature changes stress the mountings and can lead to damage. If the pipes are loosely mounted then they may buckle or move in an uncontrolled manner and take up undesirable orientations.
One way of controlling the expansion of lengths of pipe is to make the pipe in two sections which are telescopically joined by a sealing coupling. An example of such a telescopic coupling is shown in U.S. Pat. No. 5,106,129. In this document the two pipe sections are each fitted with an end collar at the ends to be connected. A tubular connecting piece with a diameter less than the inner diameter of the end collars is arranged inside the end collars to bridge the gap between the two pipe ends. The tubular connecting piece carries one sealing ring or a pair of adjacent sealing rings supported on biasing rings and retained in grooves on its external surface at each end. These sealing rings co-operate with the inner surfaces of the end collars and the sides of the retaining grooves to prevent fluid from leaking. This sealing coupling can form a seal between pipes which are angularly misaligned and permits axial movement between the parts. As the angle between the sealing coupling and the end collar changes from being parallel then the forces exerted on the sealing rings by the inner surfaces varies around their perimeters. In the case where pairs of sealing rings are used, at one point the force on the outer sealing ring is at its greatest while at the same point the force on the inner sealing ring is at its least. At the same time at a diametrically opposite point the force on the outer sealing ring is at its least and at the same point the force on the inner sealing ring is at its greatest. These asymmetric forces tend to move the rings out of alignment so that they are no longer concentric--neither with each other nor with the sealing coupling. This eccentricity is required to accommodate the increase sealing surface exposed by the misalignment. The maximum eccentricity is determined by the resilience of the biasing rings, the internal diameter of the sealing rings, the depth of the sealing rings and the depth of their retaining grooves.
The degree of misalignment which the sealing coupling can accommodate is limited by the angle a line between the seal lips in their most compressed position and the outer diameter of the grooves makes with the longitudinal axis of the sealing coupling. Any misalignment greater than this angle would cause the seal carriers to contact the inner surfaces of the end collars and damage would result. This damage would eventually cause the seal to fail.
Misalignment is also limited by the elasticity of the seal rings, as the sealing is dependent on the ability of the seal rings to be deformed by the stresses caused by the misalignment, yet at the same time be able to still exert an adequate sealing force on the larger sealing area exposed by the misalignment. A large misalignment requires a large deformation of the seal rings which requires a high degree of elasticity in the circumferential direction to allow the rings to expand. U.S. Pat. No. 5,106,129 shows various seal ring arrangements. In the arrangements which appear to be designed for harsh service, this high elasticity is provided by using split seal rings of hard, non-wearing material such as Teflon (.TM.) or metal. For an effective seal the split seal rings should be aligned such that the splits in sets of rings are not aligned. This can cause problems in service as the rings are free to rotate and may become aligned which can lead to a reduced resistance to leakage. In the case where a combination of a split sealing ring and an unsplit sealing ring is used, then the limited elasticity of the unsplit sealing ring reduces the degree of misalignment that the sealing coupling can accommodate. This is because the solid sealing ring would eventually completely lose contact with the inner surface of the end collar leading to a leak. Another disadvantage with these seals is that contain several different parts which co-operate to form a seal and that these parts have to be very accurately made and their relative sizes closely defined to ensure the desired degree of sealing. Furthermore, the effectiveness of the seal depends on the sealing rings sealing against the sides of the sealing grooves which requires a high standard of finish and hence expensive machining.
An object of the invention is to provide an expansion unit for joining pipes which can easily be adjusted to provide a seal against different pressures. A further object of the invention is to provide an expansion unit which seals against axial, radial and angular misalignments. A further object of the invention is to provide an expansion unit which uses a limited number of different sealing parts.
In accordance with the invention, these objects are accomplished in an expansion unit for joining two pipes which comprises a housing which contains two sealing arrangements each comprising a number of sealing rings of two different diameters, placed alternately side by side, with one size of ring sealing against a surface of the housing, the other size of ring sealing against the pipe, the rings having co-operating side surfaces which form a mutual seal between adjacent rings, and adjacent rings being able to move in the radial direction with respect to each other.
The expansion unit formed in accordance with the invention has a number of advantages. The use of only two types of sealing rings in each sealing arrangement reduces manufacturing and maintenance costs. It also makes it easy to manufacture expansion units with different resistance to leakage as the simple addition of more sealing rings can be used to increase the resistance to leakage.